The problems posed by substances with ozone depletion potential (ODP) were discussed in Montreal, where the protocol was signed requiring a reduction of the production and use of chlorofluorocarbons (CFCs). Amendments have been made to this protocol, requiring abandonment of CFCs and extending the regulations to cover other products, including hydrochlorofluorocarbons (HCFCs).
The refrigeration and air conditioning industry has made a considerable investment in substitution of these refrigerants, and accordingly hydrofluorocarbons (HFCs) were put on the market.
In the automobile industry, the systems for air conditioning of vehicles marketed in many countries have changed over from a chlorofluorocarbon refrigerant (CFC-12) to a hydrofluorocarbon refrigerant (1,1,1,2-tetrafluoroethane: HFC-134a), which is less harmful to the ozone layer. However, with respect to the objectives established by the Kyoto protocol, HFC-134a (GWP=1300) is considered to have a high warming power. A fluid's contribution to the greenhouse effect is quantified by a criterion, GWP (Global Warming Potential), which summarizes the warming power by taking a reference value of 1 for carbon dioxide.
As carbon dioxide is nontoxic, nonflammable and has a very low GWP, it has been proposed as a refrigerant for air conditioning systems in place of HFC-134a. However, the use of carbon dioxide has several drawbacks, notably connected with the very high pressure for its application as refrigerant in existing equipment and technologies.
Moreover, the mixture R-404A consisting of 44 wt. % of pentafluoroethane, 52 wt. % of trifluoroethane and 4 wt. % of HFC-134a is widely used as refrigerant for large areas (supermarkets) and in refrigerated transport. However, this mixture has a GWP of 3900.
Document JP 4110388 describes the use of hydrofluoropropenes of formula C3HmFn, with m, n representing an integer between 1 and 5 inclusive and m+n=6, as heat transfer fluids, in particular tetrafluoropropene and trifluoropropene.
Document WO2004/037913 discloses the use of compositions comprising at least one fluoroalkene having three or four carbon atoms, notably pentafluoropropene and tetrafluoropropene, preferably having a GWP of at most 150, as heat transfer fluids.
Document WO 2006/094303 discloses an azeotropic composition containing 7.4 wt. % of 2,3,3,3-tetrafluoropropene (HFO-1234yf) and 92.6 wt. % of difluoromethane (HFC-32). This document also discloses quasi-azeotropic compositions containing from 1 to 57 wt. % of 2,3,3,3-tetrafluoropropene and from 43 to 99 wt. % of difluoromethane.
A heat exchanger is a device for transferring thermal energy from one fluid to another, without mixing them. The thermal flux passes through the exchange surface that separates the fluids. Mostly this method is used for cooling or heating a liquid or a gas that cannot be cooled or heated directly.
In compression systems, heat exchange between the refrigerant and the heat sources takes place via heat-transfer fluids. These heat-transfer fluids are in the gaseous state (the air in air conditioning and direct-expansion refrigeration), liquid (water in domestic heat pumps, glycol solution) or two-phase.
There are various transfer modes:                the two fluids are arranged in parallel and go in the same sense: co-current mode (antimethodical);        the two fluids are arranged in parallel but go in the opposite sense: countercurrent mode (methodical);        the two fluids are positioned perpendicularly: crossed-current mode. The crossed current can have co-current or countercurrent tendency;        one of the two fluids makes a U-turn in a wider pipeline, which the second fluid passes through. This configuration is comparable to a co-current exchanger on half its length, and to a countercurrent exchanger for the other half: pin-head mode.        
The applicant has now discovered that binary compositions of 2,3,3,3-tetrafluoropropene and difluoromethane are particularly advantageous as heat transfer fluid in compression-type refrigeration systems with exchangers operating in countercurrent mode or in crossed-current mode with countercurrent tendency.
Thus, these compositions can be used as heat transfer fluid in heat pumps, optionally reversible, up to a heating temperature of 95° C., in air conditioning, industrial air conditioning (paper, rooms for servers), in mobile domestic air conditioning, in domestic refrigeration and freezing, employing compression systems with exchangers in countercurrent mode or in crossed-current mode with countercurrent tendency.